Definition and Overview
A debt security is a financial instrument that represents a creditor relationship between the holder of the instrument and the issuer, in which the issuer borrows money from the holder and agrees to repay the principal amount borrowed on a specified future date, called the maturity date, and to pay periodic interest, called the coupon, at a specified rate or according to a specified formula during the life of the instrument.
The holder of a debt security is a creditor of the issuer, not an owner. The holder has a contractual legal claim to the promised cash flows of principal and interest but does not share in the growth, profits, or residual value of the issuer's business in the way that an equity holder does.
Debt securities are issued by a vast range of entities including sovereign governments, sub-sovereign governmental entities such as states, municipalities, and government agencies, supranational organisations such as the World Bank and International Monetary Fund, corporations of every size and credit quality, financial institutions including banks and insurance companies, and special purpose vehicles created to hold pools of financial assets in structured finance transactions.
The global debt market is the largest financial market in the world, significantly exceeding the global equity market in total outstanding value and daily trading volume, and encompasses instruments ranging from the simplest overnight Treasury bill to the most complex structured credit product.
Understanding debt securities thoroughly is foundational to securities industry practice.
Fixed income instruments are central to asset allocation, portfolio management, retirement income planning, corporate finance, and monetary policy.
They appear throughout the securities examination framework and underpin many of the most important analytical concepts in investment management including duration, yield, credit analysis, and the relationship between price and interest rates.
The Essential Features of a Debt Security
Every debt security, regardless of its specific structure, issuer, or market, is defined by a set of essential features that determine its cash flow profile, risk characteristics, and valuation.
The principal amount, also called the face value, par value, or notional amount, is the amount that the issuer promises to repay to the holder at maturity. For most bonds issued in the United States the standard face value is one thousand dollars per bond, though this varies for different instrument types. The principal is the benchmark for calculating the coupon payment and for expressing the bond's price as a percentage of par value, the standard convention in professional bond markets.
The coupon rate is the annual interest rate expressed as a percentage of the principal that determines the periodic interest payments the issuer makes to the holder. A bond with a one thousand dollar face value and a five percent annual coupon rate pays fifty dollars in interest per year, typically in two semi-annual payments of twenty-five dollars each for bonds following the standard US market convention. The coupon rate is set at the time of issuance and remains fixed for the life of a fixed-rate bond, though variable-rate or floating-rate bonds have coupons that reset periodically based on a reference rate.
The maturity date is the date on which the issuer is obligated to repay the full principal amount to the holder. Debt securities span an enormous range of maturities, from overnight instruments such as commercial paper and repurchase agreements through short-term instruments with maturities of one year or less, medium-term instruments with maturities of one to ten years, long-term instruments with maturities of ten to thirty years, and ultra-long instruments including some sovereign bonds with maturities of fifty or one hundred years. The maturity of a debt security is one of the primary determinants of its interest rate risk and its place in portfolio construction.
The yield is the total return a holder will earn on a debt security if they purchase it at the current market price and hold it until maturity, accounting for all coupon payments received and the difference between the purchase price and the face value repaid at maturity. Yield is the most important single measure of a debt security's return and is the primary basis on which debt securities are compared, priced, and valued in professional markets. Because price and yield move inversely, a bond purchased at a discount to face value will have a yield above its coupon rate while a bond purchased at a premium will have a yield below its coupon rate.
The credit quality of the issuer is the assessment of the probability that the issuer will fulfil all promised payments of principal and interest in full and on time. Credit quality is formally assessed by rating agencies including Moody's, S&P, and Fitch and expressed through the rating scales described in the Bond Rating article. It is the primary non-interest-rate determinant of a debt security's yield, with lower credit quality issuers required to offer higher yields to compensate investors for bearing greater default risk.
Types of Debt Securities by Issuer
The classification of debt securities by issuer type is one of the most important organisational frameworks in fixed income and reflects meaningful differences in credit quality, tax treatment, regulatory status, and the economic and institutional factors that drive each issuer's borrowing.
United States Treasury securities are debt obligations of the federal government backed by the full faith and credit of the United States, making them the highest credit quality instruments in the dollar-denominated fixed income universe and the benchmark against which all other dollar-denominated debt securities are priced. Treasury securities are issued in three primary structures: Treasury bills with maturities of one year or less issued at a discount to face value with no explicit coupon payment, Treasury notes with maturities of two to ten years paying semi-annual coupons, and Treasury bonds with maturities of ten to thirty years paying semi-annual coupons. Treasury Inflation-Protected Securities adjust both their principal value and their coupon payments for changes in the Consumer Price Index, providing explicit inflation protection unavailable from nominal Treasury securities. Interest on Treasury securities is subject to federal income tax but exempt from state and local income taxes. Treasury securities are the most liquid fixed income instruments in the world, with daily trading volumes in the trillions of dollars.
Agency securities are debt obligations of government-sponsored enterprises and federal agencies including Fannie Mae, Freddie Mac, Ginnie Mae, the Federal Home Loan Banks, and the Farm Credit System. As described in the Agency Bond article, Ginnie Mae securities carry the explicit full faith and credit guarantee of the US government while other agency securities carry an implied guarantee that has been affirmed in practice but is not contractually binding. Agency securities include both conventional debt obligations paying regular coupons and mortgage-backed securities representing pools of residential mortgage loans whose principal and interest payments flow through to investors.
Municipal securities are debt obligations of state and local governmental entities including states, cities, counties, school districts, transit authorities, water and sewer systems, and various special purpose governmental bodies. The most important feature of most municipal securities for US investors is the federal income tax exemption on interest income, which makes their after-tax yield competitive with higher-yielding taxable securities for investors in higher tax brackets. Municipal bonds are issued in two primary structures: general obligation bonds backed by the full taxing power of the issuing governmental body, and revenue bonds backed by the revenues generated by a specific project or facility such as a toll road, airport, hospital, or water system.
Corporate bonds are debt obligations of private corporations, ranging from investment grade bonds issued by large financially strong companies with stable cash flows to high yield bonds issued by smaller, more leveraged, or less financially strong companies that must offer higher yields to attract investors willing to bear the greater default risk. Corporate bonds are subject to federal, state, and local income taxes on interest received, distinguishing them from Treasury and most municipal bonds in their tax treatment. Corporate bonds span a wide range of maturities, structures, and features, including callable bonds that give the issuer the right to redeem the bonds before maturity, convertible bonds that give the holder the right to convert the bonds into equity at a specified conversion ratio, and floating rate notes whose coupons reset periodically based on a reference rate such as SOFR.
International and foreign bonds include debt securities issued by foreign governments and corporations, sovereign bonds issued in local currency and Eurobonds issued in currencies other than the domestic currency of the country in which they are issued. International fixed income offers diversification benefits through exposure to different economic cycles, interest rate environments, and credit dynamics than those affecting the US market, but introduces currency risk, country risk, and potential limitations on legal recourse that add layers of complexity to the credit analysis and risk management of these instruments.
Structured finance securities include asset-backed securities, mortgage-backed securities, collateralised loan obligations, and other debt instruments whose cash flows are derived from pools of underlying financial assets rather than from the direct obligation of a single corporate or governmental issuer. The credit quality and cash flow profile of structured finance securities depends on the characteristics of the underlying asset pool, the structural protections and credit enhancements incorporated into the transaction, and the legal framework governing the special purpose vehicle that holds the assets and issues the securities.
Fixed Rate vs Floating Rate Debt Securities
One of the most important structural distinctions among debt securities is between fixed rate and floating rate instruments, which differ fundamentally in how their coupon payments are determined and in their exposure to changes in market interest rates.
Fixed rate debt securities pay a coupon that is set at the time of issuance and remains constant throughout the life of the instrument regardless of subsequent changes in market interest rates. This certainty of cash flow is one of the primary advantages of fixed rate bonds for income-oriented investors who need predictable periodic payments. However the fixed coupon rate means that fixed rate bonds are fully exposed to interest rate risk: when market interest rates rise above the fixed coupon rate, the bond's price falls to maintain the yield at the new market rate. The longer the remaining maturity of the bond, the greater its price sensitivity to interest rate changes, as measured by the duration concept described in the Duration article.
Floating rate debt securities, also called variable rate or adjustable rate instruments, pay coupons that reset periodically at a spread over a reference rate, most commonly the Secured Overnight Financing Rate, which replaced LIBOR as the primary US dollar reference rate following the LIBOR transition completed in 2023. A floating rate note might pay a coupon of SOFR plus one hundred and fifty basis points, resetting quarterly. Because the coupon adjusts to reflect current market rates at each reset date, floating rate notes have much lower price sensitivity to interest rate changes than fixed rate bonds and tend to trade close to par value under most market conditions. The trade-off is uncertainty about future coupon income, since the rate received depends on future levels of the reference rate.
Zero coupon bonds are a special category of fixed income instrument that pays no periodic interest during their life but is issued at a substantial discount to face value, with the investor's entire return coming from the accretion of the discount to par value at maturity. A zero coupon bond with a face value of one thousand dollars and a ten-year maturity might be issued at a price of six hundred and fourteen dollars if the prevailing ten-year yield is five percent. The investor receives no cash payments for ten years but receives one thousand dollars at maturity, earning a five percent annual return through the appreciation from six hundred and fourteen to one thousand dollars over the holding period. Zero coupon bonds have the highest duration of any bond structure for a given maturity, making them the most price-sensitive debt instruments for any given maturity, and they are extensively used in liability-driven investment strategies to match specific future cash obligations with certainty.
The Priority of Claims in Debt Securities
The position of a debt security in the issuer's capital structure determines the priority of the holder's claim on the issuer's assets and cash flows in the event of financial distress or bankruptcy, and this priority is a critical determinant of the instrument's credit risk and appropriate yield.
Senior secured debt sits at the top of the capital structure with the highest priority claim on the issuer's assets and cash flows. In the event of default, senior secured creditors have the right to seize and liquidate specific collateral pledged to secure their loans before any other creditors receive any recovery. Because of their priority position and collateral protection, senior secured debt instruments typically carry the highest credit rating within a given issuer's capital structure and offer the lowest yield relative to other debt instruments of the same issuer.
Senior unsecured debt has a priority claim on the issuer's assets after secured creditors have been satisfied but before subordinated creditors and equity holders receive any recovery. Senior unsecured bonds represent the most common form of corporate bond and are the primary benchmark for assessing an issuer's credit quality in the investment grade and high yield markets. They offer higher yields than senior secured debt of the same issuer to compensate for the absence of specific collateral protection.
Subordinated debt, also called junior debt or mezzanine debt in certain contexts, has a lower priority claim on the issuer's assets than senior debt. In bankruptcy, subordinated creditors receive no recovery until senior creditors have been paid in full. The greater risk of loss in default is compensated by higher yields relative to senior debt of the same issuer. Subordinated debt is commonly issued by financial institutions, which use it as a component of their regulatory capital structures, and by highly leveraged corporate issuers in connection with leveraged buyout financings.
Convertible bonds occupy a hybrid position in the capital structure, combining the characteristics of a debt security with an embedded option allowing the holder to convert the bonds into equity at a predetermined conversion ratio. As debt instruments, convertible bonds have a senior claim on the issuer's assets relative to equity holders. As instruments with embedded equity optionality, they offer lower yields than comparable non-convertible bonds because the conversion option has value to holders who believe the issuer's equity may appreciate significantly during the life of the bond.
Price, Yield, and the Inverse Relationship
The relationship between the price of a debt security and its yield is one of the most fundamental and most examined concepts in fixed income markets, and understanding it intuitively as well as mathematically is essential for any securities industry professional.
The price of a debt security at any given moment represents the present value of all future cash flows, discounted at the yield that the market requires for securities of equivalent credit quality, maturity, and other characteristics. When market yields rise, the discount rate applied to future cash flows increases, reducing their present value and therefore the price of the bond. When market yields fall, the discount rate decreases, increasing the present value of future cash flows and therefore the price of the bond. This inverse relationship between price and yield is mathematically precise and universally applicable to all fixed income instruments.
The magnitude of the price change for a given change in yield depends on the maturity and coupon rate of the bond, with longer-maturity and lower-coupon bonds experiencing larger price changes for a given yield change than shorter-maturity and higher-coupon bonds. This price sensitivity is captured formally by the duration concept, which measures the weighted average time to receipt of the bond's cash flows and serves as a linear approximation of the percentage price change for a given change in yield. Convexity refines this approximation by accounting for the curvature in the price-yield relationship that makes duration an increasingly imprecise estimate as yield changes become larger.
Credit Risk and Yield Spreads
Credit risk is the risk that the issuer of a debt security will fail to make promised payments of principal or interest in full and on time, resulting in default. The yield spread, the difference between the yield on a corporate or other non-government debt security and the yield on a Treasury security of comparable maturity, compensates investors for bearing the credit risk of the non-government issuer. Wider spreads indicate higher perceived credit risk while narrower spreads indicate lower perceived risk, and spread movements over time reflect changing assessments of default probability and recovery expectations.
The credit spread can be decomposed into several components including the expected loss from default, calculated as the probability of default multiplied by the expected loss given default, a risk premium compensating investors for the uncertainty around the expected loss and the potential correlation of default with adverse economic conditions, and a liquidity premium compensating investors for the lower liquidity of corporate bonds relative to Treasury securities. Separating these components is analytically important because they respond differently to changes in economic conditions and market sentiment.
Examination Relevance and Key Takeaways
Debt securities are among the most comprehensively tested topics across the SIE, Series 7, and Series 65 examinations. Candidates must understand the essential features of debt securities including principal, coupon, maturity, and yield, the major categories of debt securities by issuer type including Treasury, agency, municipal, and corporate securities and their distinctive characteristics, the structural distinction between fixed rate and floating rate instruments, the priority of claims in the capital structure and its implications for credit risk and yield, and the inverse relationship between price and yield that is the foundation of all fixed income price analysis.
The core points to retain are these: a debt security represents a creditor relationship in which the holder lends money to the issuer in exchange for promised payments of interest and principal; the holder is a creditor with contractual claims rather than an owner with residual claims; the major issuer categories are Treasury, agency, municipal, and corporate, each with distinctive credit quality, tax treatment, and market characteristics; fixed rate bonds have constant coupons and full exposure to interest rate risk while floating rate notes reset their coupons periodically and have minimal price sensitivity to rate changes; zero coupon bonds pay no periodic interest and have the highest duration of any bond structure; the position of a debt security in the capital structure from senior secured through subordinated determines its priority in default and therefore its credit risk and required yield; and bond prices and yields always move in opposite directions because a bond's price is the present value of its future cash flows discounted at the prevailing market yield.
